It is common to distribute paging messages to individual paging receivers (pagers) by using existing radio or television stations as repeaters. The messages originate at a paging terminal which receives information to be sent to the pagers from, for example, a telephone call made via the public switched telephone network. The paging message is then formatted into a paging signal suitable for radio transmission.
In wide area systems, such as may be used to cover the United States, the paging signal is first transmitted on a satellite uplink to an orbiting satellite. The satellite contains a radio transceiver which decodes and remodulates the uplink signal for transmission on a number of downlink signals. The downlink signals are then received by terrestrial repeater stations. These terrestrial repeater stations are typically located at the site of an existing commercial frequency modulated (FM) radio or television broadcaster. The repeater station receives the downlink paging signals, reformats them, and then broadcasts them on one of several radio frequencies to pagers located within range of the repeater station. These paging receivers may be of the type carried by persons or may be mounted in or on vehicles.
The United States' Federal Communications Commission (FCC) has provided for so-called Subsidiary Communications Authorization (SCA) programming in which a licensed FM radio station may broadcast multiple secondary programs of audio or data signals. The secondary signals are broadcast using subcarriers located at, for example, 67 or 92 kilohertz (kHz) offset from the licensed radio carrier frequency.
Such SCA broadcasts are commonly used for carrying paging signals from the existing radio tower to pagers located within the range of the FM station. Since the paging messages are carried as an SCA-type subcarrier signal, they do not interfere with the primary commercial FM broadcast of audio programs.
With this arrangement, it is not possible to deliver messages to pagers that are located in areas which are not covered by existing FM stations. It is therefore believed that such paging systems have a coverage area limited to locations in which licensed FM radio stations are operating. Such paging systems have not been applied in certain contexts as a result.
For example, in the instance of a cargo tracking system, it might be possible to install pagers in each cargo container for receiving information from a central control point as the containers travel via trucks, ships and/or trains from place to place. It is common for cargo containers to be loaded onto wide ranging vehicles such as a ship that is travelling on the open sea or a cross-continental train that is moving through a remote area. However, an SCA subcarrier paging system as has been described above could not be reliably used when the vehicle travels outside of the range of the FM broadcast stations.
Although each container could be equipped with a satellite dish and receiver, such satellite equipment tends to be fairly expensive. In addition, the prospect of equipping each container with a satellite antenna would typically be undesirable. Such satellite equipment is fairly large and protrudes outside of the container, and would therefore be prone to breakage.
What is needed is a way to enable a paging system to reach or otherwise communicate using FM repeater type subcarrier signalling even when such pagers are located in remote areas such as in the wilderness or the ocean which are outside of the range of commercial FM stations. At the same time the system should avoid the high cost of direct satellite receiving equipment for each container.